Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>
To:     Celeste Liu <coelacanthushex@gmail.com>,
        Palmer Dabbelt <palmer@rivosinc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>, Guo Ren <guoren@kernel.org>,
        =?utf-8?B?QmrDtnJu?= =?utf-8?B?IFTDtnBlbA==?= 
        <bjorn@rivosinc.com>, Conor Dooley <conor.dooley@microchip.com>,
        linux-riscv@lists.infradead.org
Cc:     linux-kernel@vger.kernel.org,
        Celeste Liu <CoelacanthusHex@gmail.com>,
        Felix Yan <felixonmars@archlinux.org>,
        Ruizhe Pan <c141028@gmail.com>,
        Shiqi Zhang <shiqi@isrc.iscas.ac.cn>
Subject: Re: [PATCH v2] riscv: entry: set a0 prior to
 syscall_enter_from_user_mode
In-Reply-To: <20230718162940.226118-1-CoelacanthusHex@gmail.com>
References: <20230718162940.226118-1-CoelacanthusHex@gmail.com>
Date:   Tue, 18 Jul 2023 21:35:37 +0200
Message-ID: <87pm4p2et2.fsf@all.your.base.are.belong.to.us>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

Celeste Liu <coelacanthushex@gmail.com> writes:

> When we test seccomp with 6.4 kernel, we found errno has wrong value.
> If we deny NETLINK_AUDIT with EAFNOSUPPORT, after f0bddf50586d, we will
> get ENOSYS instead. We got same result with 9c2598d43510 ("riscv: entry: =
Save a0
> prior syscall_enter_from_user_mode()").
>
> After analysing code, we think that regs->a0 =3D -ENOSYS should be advanc=
ed before
> syscall_enter_from_user_mode to fix this problem. In __seccomp_filter, wh=
en
> seccomp rejected this syscall with specified errno, they will set a0 to r=
eturn
> number as syscall ABI, and then return -1. This return number is finally =
pass
> as return number of syscall_enter_from_user_mode, and then is compared
> with NR_syscalls after converted to ulong (so it will be ULONG_MAX).
> The condition syscall < NR_syscalls will always be false, so regs->a0 =3D=
 -ENOSYS
> is always executable. It covered a0 set by seccomp, so we always get
> ENOSYS when match seccomp RET_ERRNO rule.

Isn't something like...

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f910dfccbf5d..15a8b4898a6c 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -297,7 +297,7 @@ asmlinkage __visible __trap_section void do_trap_break(=
struct pt_regs *regs)
 asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *r=
egs)
 {
 	if (user_mode(regs)) {
-		ulong syscall =3D regs->a7;
+		long syscall =3D regs->a7;
=20
 		regs->epc +=3D 4;
 		regs->orig_a0 =3D regs->a0;
@@ -306,7 +306,7 @@ asmlinkage __visible __trap_section void do_trap_ecall_=
u(struct pt_regs *regs)
=20
 		syscall =3D syscall_enter_from_user_mode(regs, syscall);
=20
-		if (syscall < NR_syscalls)
+		if (syscall > -1 && syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
 		else
 			regs->a0 =3D -ENOSYS;


...easier to read?


Bj=C3=B6rn